The traffic management
isn't the traditional kind that imposed speed cuts at various times. I've only heard of a couple of people who seem to get some strange effect from it - the rest who were in doubt like you and those of us around when the earlier traditional version was dropped don't notice it and often say so.
The CS problem has been a slow response on the phone and to tickets. That problem seems to be decreasing, as we were promised it would now that the second call centre and newly recruited staff are getting up to speed. The reps on the in-house forms are very good, and visit these forums as well though not posting nearly every day as they used to.
For my first FTTC when I left O2 LLU I chose IDNet, that have a similar reputation to Zen but are smaller. After my first 12 months I moved to Plusnet as IDNet were very expensive and I couldn't see what I gained from paying more. Since when I have upgraded twice, the second time being eleven months into my 18-month term and meaning I had to restart the eighteen months. That didn't bother me in the least.
I had big trouble for years on ADSL2+ due to intermittent huge noise on the line. That's a different story but was a major factor in going for FTTC. I believed the problem was the cables from the cabinet to the exchange, which of course FTTC bypasses. Fortunately I was right - but choosing IDNet was party caused by the worry that I might need good CS if I'd been wrong. At the time Plusnet did not have FTTC.
Re your own connection, from the posts on here I'm not sure the Zen CS for non-business customers is quite as caring as it might be, but it's not something I could easily substantiate. Suffice to say I don't think I would go there myself. On the other hand at one time I would never have looked at Plusnet - but things with them have changed dramatically since those days.
I think you should look at AAISP and Plusnet, but on your Market 1 exchange you will probably find Plusnet prices £7.50pm higher than the headline prices.
Another alternative is xilo
. A large number of ex-ADSL24 customers fed up since COMS took them over have joined them, as have some ex-O2/Be deserters from Sky. The CS has a magical reputation and the broadband service seemingly fine.